1. Field
The present invention relates to an apparatus for acceleration of a beam of charged particles along a linear trajectory in a linear accelerator (linac). More particularly, the present invention is related to an Interdigital (or Wideröe) linac consisting of a linear array of electrodes, or drift tubes, that can be excited with radio frequency (rf) power to produce electric fields in the gaps between the electrodes that alternate in direction from adjacent gaps in a manner suitable for acceleration of protons, deuterons, and heavier ions.
2. Background
Particle accelerators are machines built for the purpose of accelerating electrically charged particles to kinetic energies sufficiently high to produce certain desired nuclear reactions, ionization phenomenon, and/or materials modification processes. Typically, charged particles from an ion source are collimated into a “beam” and injected into accelerating structures, where they follow certain trajectories under the influence of bending, steering, focusing and accelerating fields until they have reached the required energy. At this point, the beam is typically extracted from the accelerator system and directed onto a “target”, where the desired reactions occur. The by-products of these reactions can be used for scientific, medical, industrial and military applications.
Linear accelerators (linacs) represent one of the main technologies for the acceleration of charged particles (atomic ions) from their sources (ion sources) to the desired particle energy, or to particle energies where other types of accelerators, such as synchrotrons (circular accelerators), are preferred. For protons, this often encompasses the energy range from 30 kilo-electron-volts (keV) to hundreds of million-electron-volts (MeV), or a velocity range from about 0.008 to about 0.8 times the velocity of light.
Linacs generally involve evacuated, metallic cavities or transmission lines, filled with radio-frequency electromagnetic energy waves that result in strong alternating electric fields that can accelerate charged particles. Linac art is categorized by the properties of the rf waves, yielding two types of linacs, namely standing wave linacs and traveling wave linacs.
Alternatively, linacs may be classified according to the particle velocities that they accommodate. Generally speaking, standing wave linacs are used for particle velocities less than half the velocity of light (low beta linacs). Both standing wave and traveling wave linacs are used for higher velocities (high beta linacs). At velocities close to that of the velocity of light, traveling wave linacs predominate.
Common standing wave linac structures include the radio frequency quadrupole (RFQ) linac structure, which has become common in the lowest-velocity end of linacs, the interdigital, or Wideröe linac, which is sometimes used for acceleration of low-energy heavy ions, the drift tube linac (DTL) structure, commonly used for middle-velocity linacs, and the coupled cavity linac (CCL) structure, typical of high-velocity standing wave linacs.
Linacs accelerate charged particles along nominally straight trajectories by means of alternating electric fields in gaps between linear arrays of electrodes located inside evacuated cavities. The alternating electric fields in these evacuated metallic cavities or transmission lines result from the excitation of electromagnetic cavity modes with radio frequency electromagnetic energy. The electrode spacing is arranged such that particles arrive at each gap between electrodes in an appropriate phase of the electric field to result in acceleration at each gap.
The capabilities of conventional linacs for accelerating high beam currents at low energies are severely limited by the available strengths of the conventional magnetic focusing elements, used to keep the beam diameters small enough to enable efficient interactions with the rf electric accelerating fields. In the development of linac technology, there have been numerous attempts to utilize electric fields for the focusing forces, which, unlike magnetic fields, are independent of particle velocity and promise superior performance at lower particle velocities. Both static electric quadrupole fields and time-dependent (rf) electric quadrupole fields have been considered for this role.
In the early 1970's the revolutionary idea of “spatially uniform strong focusing” was introduced, which offers the capability of simultaneously focusing, bunching and accelerating intense beams of charged particles with rf electric fields in one compact structure. This subsequently became known as the radio frequency quadrupole (RFQ) linac structure. RFQ linacs represent the best transformation between the continuous beams that come from ion sources and the bunched beams required by most linear accelerators. Their forces, being electric, are independent of particle velocity, allowing them to focus and bunch beams at much lower energies than possible for their magnetically focused counterparts. Their capture efficiency can approach 100% with minimal emittance growth. RFQ linacs have made a major impact on the design and performance of proton, deuteron, light-ion, and heavy-ion accelerator facilities. They have set new performance standards for accelerators and in so doing have earned a role in most future proton and other ion accelerators.
However, RFQ linacs are not without limitations. In all RFQ linac structures, the acceleration rate is inversely proportional to the particle velocity. Therefore, at some point in the process of particle acceleration, the acceleration rate drops to the point where some change in the acceleration process is desired. Unfortunately, in the conventional RFQ structure, there are no changes that can be made to the basic structure to rectify the inherent deterioration of the acceleration rate that occurs with higher velocities. As a result, for all but the lowest energy applications, an RFQ linac must be followed by a different accelerating structure, such as a magnetically focused drift tube linac (DTL), which offers higher acceleration rates in the energy range just beyond the practical limits of the RFQ structure up to velocities as high as half the speed of light. However, the magnetic focusing at the low-energy end is generally weaker than the electric focusing utilized in the RFQ structure. Consequently, matching the beam from an electrically focused RFQ linac into a magnetically focused DTL linac—often requiring several additional focusing and bunching elements as well as beam diagnostics equipment to manage the transition—tends to be too complex and expensive for most commercial applications.
U.S. Pat. No. 5,113,141, entitled “Four-Fingers RFQ Linac Structure”, to Swenson, also the inventor of the subject technology herein, introduced an improved RFQ linac structure to extend the useful energy range of the conventional RFQ linac structure. The invention introduced a new degree of freedom into the system by configuring the structure as individual, four-finger-loaded acceleration/focusing cells, the orientation of which would be chosen to optimize performance. This new degree of freedom made the acceleration periodicity independent of the focusing periodicity, thus allowing the operating frequency to be raised as needed to enhance the acceleration rate without jeopardizing the required focusing action.
U.S. Pat. No. 5,523,659, entitled “Radio Frequency Focused Drift Tube Linear Accelerator”, also to Swenson, introduced a new linac structure that combined the superior focal properties of the RFQ with the superior acceleration properties of the DTL linac. This structure provided strong rf focusing and efficient rf acceleration for particles at velocities beyond that which is practical for the RFQ structure. It provided a way to incorporate rf focusing into the drift tubes of a drift tube linear accelerator excited in the TM010 rf cavity mode. This rf focused drift tube (RFD) linac structure offered the advantages of lowering the maximum energy of the RFQ to the range where it was more efficient, and extending the energy range of the combination far beyond the capabilities of the RFQ linac. The RFD linac structure, combined with a short RFQ section, offered efficient acceleration of light-ions, such as protons and deuterons, to output energies from a few MeV to 100 MeV, at radio frequencies of 200 MHz and above.
Most heavy-ion linacs, however, operate in the frequency range of 20–50 MHz. In this frequency range, DTL structures, including the RFD linac structure, become very large in diameter; for example 10 meters in diameter for a frequency of 20 MHz. For this reason, most heavy-ion linacs begin with some form of interdigital linac structure, which is modest in size—less than 1 meter in diameter—at those frequencies. As used herein, “heavy ion” refers to all ions that are heavier than the lightest ion, namely the proton. Examples of heavy ions include deuterons and ions of boron, lithium, carbon, etc. as will be understood by those skilled in the art.
It would be desirable for a linac structure to extend the remarkable rf electric quadrupole focusing properties of the RFQ linac to some form of interdigital linac, suitable for use at the lower frequencies typically used for heavy-ion acceleration.
The present invention for an rf focused interdigital linac, or “RFI linac”, provides a way to incorporate rf focusing into the drift tubes of an interdigital linear accelerator excited in a TE110-like rf cavity mode. The resulting structures are more compact and energy efficient than structures based on the TM010 rf cavity mode. The RFI linac extends the performance of the RFQ, or other, linac structures by accelerating the small diameter, tightly bunched beams that come from RFQ, or other, linacs to higher energies.
The terms TM010, TM010-like, TE110, and TE110-like describe rf electric and magnetic field configurations in cylindrical cavities and are well known and understood by those skilled in the art. The terms TM010 and TE110 are well defined for empty cylindrical cavities, where the TM010 mode is the lowest frequency rf cavity mode having a transverse magnetic field, and the TE110 mode is the lowest frequency rf cavity mode having a transverse electric field. The introduction of additional structure within these cylindrical cavities—in this case, the drift tubes and their supports, which are essential to the acceleration process—perturbs the pure cylindrical cavity modes, resulting in what those skilled in the art refer to as TM010-like and TE110-like rf cavity modes.